Pulse-code selective communication system



5 Sheets-Sheet l Original Filed Sept. 18, 1945 INVENTOR.

MclLwAlN KNOX ATTO Y.

Dec. 29, 1953 K. McxLwAlN PULSE-CODE SELECTIVE COMMUNICATION vSYSTEM original Filed sept. 18, 1945 5 Sheets-Sheet 3 INVENTOR.

y KNOX MclLWAiN A TORNEY,

Dec. 29, 1953 K. MclLwAlN PULSE-CODE SELECTIVE COMMUNICATION SYSTEM 5 Sheets-Sheet 4 Original Filed Sept. 18, 1945 waz-ac3 INVENToR.

KN X MclLWAlN Wy A oRNEY.

5 Sheets-Sheet 5 Original Filed Sept. 18, 1945 Patented Dec. 29, 1953 PULSE-CODE SELEC'li/'E COMMUNICA- TION SYSTEM Knox Mcllwain, New York, N. Y., assigner to Hazeltine Research, inc., Chicago, Ill., a corporation of Illinois 12 Claims. 1

The present invention relates to pulse-code selective communication systems and, particularly, to such systems for communication between two spaced Wave-signal stations at least one of which is carried by a mobile object. In greater particularity, the invention relates to a pulsecode selective communication system particularly suited for the navigation, regulation, and control of traffic ovv either or both along a traiic route or among a plurality of trahc zones available to mobile objects such, for example, as air craft, seagoing vessels, and trains. The present invention is a continuation of abandoned arp-- plication Serial No. 617,020, filed September 18, i945, and entitled Pulse-Code Selective Communication System.

Under the presently used plan of commercial aircraft trahie control, flights along prescribed routes are established and navigated either or both by visible landmarks or by radio range or beacon systems. Altitude separation is used in cross-country night and when stacked at an airport with other aircraft awaiting permission to land. In cross-country night, several aircraft may be permitted to fly at the same altitude level if travelingin the same direction and adequately spaced by their relative departure schedules.

Any condition unexpectedly arising during flight which may upset a night schedule approved by a tramo-control dispatcher requires an immediate reappraisal of numerous other approved iiight schedules which, under the changed condition, may become conicting. Since this may require immediate modification of numerous other night schedules then in progress, a heavy burden and responsibility is thrust upon an airport traffic-control dispatcher, perhaps necessitating cti-ordinated action with other dispatchers at distant airports, to make immediate and unerring decisions and quickly to issue whatever orders are necessary to effect the necessary flight schedule modifications. Under this plan, all responsibility falls upon one or more trahie-control dispatchers and little responsibility remains with the pilot of an aircraft except to maintain his night in conformity with the approved iight schedule. Heretoiore, the number of commercial aircraft normally in night at any given time and in any given locality has been so relatively small that this trahie-control procedure has in general been satisfactory. However, with the contemplated substantial increase in the number of aircraft, both commercial and private, which will come into future use within a few years, it has been suggested that this procedure must require substantial revision and modification if it is to have any semblance of utility.

In making any such revisions to current aircraft traffic-control procedure, it would be highly desirable that the responsibility for the major burden of actual flight maneuver be cast upon an aircraft pilot much as the responsibility for safe travel along an automobile highway is cast upon the driver of an automobile. In so doing, it would be desirable that the pilot be assured of remaining by instrument control on an accurately established prescribed course and in a given one of a plurality of accurately estabe lished unvarying altitude zones of night, thus to provide an arrangement analogous to elevated highways for vehicular traino. This would permit an individual altitude zone to be permanently assigned to air traffic moving toward each given point of the compass and thereby permit indiscriminate crossings of flight routes without creating a hazardous flight condition. While in his given altitude zone, it would further be desirable that the pilot be aware even during extremely poor visibility of the relative position and direction of flight of every other aircraft moving Within a given range of him and within the same altitude Zone. The pilot thereby would be enabled to maintain an adequate safe spac ing between his own craft and others following the same route and would further be enabled, Without danger of collision, to establish his own selected holding circle on either side of the prescribed route, as Where the trahie became jammed-up for some reason on the course ahead of him. In this desired arrangement, however, the pilot should not be confused by Warnings of the presence of aircraft in other altitude zones, since these cannot create a hazardous night condition for him, except inthe case of an aircraft Which contemplated moving up or down from an adjacent altitude zone into his own altitude zone. These he should become aware oi in suicient time prior to such movement that he can take proper steps himself to avoid a hazardous night condition and can Warn the approaching aircraft of the dangerous condition which may arise from the maneuver. All such altitude zones should preferably be established and maintained automatically by instrument Without resort to the use of a manually set barometer. When this is done, all altitude zones are accurately established without reference to changing barometric pressures at a given energy having two relatively variable wave-form portions varying with those of the modulationsignal energy of the interrogator. In one form of the invention, the derived signal energy has a wave form corresponding to that of the modulation-signal energy and including the two variable wave-form portions thereof, for example, a pulse wave form in which the leading edges oi two successive pulses thereof have a spacing varying in correspondence with that of the modulation signal. This means comprises a receiver Vi having an input circuit coupled to an antenna it and forming one unit of a transponder I9.

The communication system, in particular the transpondor le thereof, is provided with rneans including a control means at the other station having a cycle of operation initiated in a predetermined time relationship with the occurrence of one variable wave-form portion of the derived signal energy, for example. the leading edge of a rst derived pulse, for utilizing the derived signal energy only when the other variable waveform portion ofthe derived signal energy, for example, the leading edge of a successive derived pulse, has a relative amount or" variation lying within a selectable range of values established by the control means as designating a selectable one of the plurality of traic zones. The control means comprises a decoder un-'t 2D, the arrangement and operation of which will be more fully considered hereinafter, having an input circuit coupled to an output circuit of the receiver Il. An altimeter 2l controls the operation of the unit 2d. The utilizing means mentioned includes a reply-signal generator 22 having an input circuit coupled to the output circuit of the decoder 2i? and having an output circuit coupled to an input circuit of an amplifier 23, and to a gaincontrol circuit of the receiver Il. The output circuit of the aniplier 23 is coupled to a modulation input circuit of a wave-signal generator 24 to modulate the wave signal generated therein. The output circuit of the generator 2d is coupled to an antenna system 25, Essentially, the units 22, 23 and 24 comprise a transmitter included in the transponder I9 for transmitting a modulated wave-signal reply, which may in some cases be coded for identiiication or communication pur poses.

The communication system also includes means at the one station, that including unit i9, for receiving the reply wave signal transmitted by the transponder le and for utilizing the modulation components thereof to provide an indication which may simply be an indication that a reply has been received, as when the output of this receiver is applied to a pair of earphones, or an indication of either or both the distance between the two stations or the direction of the other station from the one station. This means comprises a receiver system Z5, hereinafter designated for convenience as a responsen which is shown as of the type for providing an indication of both the distance and the direction ci the transponder iii from the interrogator le and respcnser 26. For this purpose, the responser 26 includes a pair of directive antenna systems 21, 2S having slightly overlapping directional characteristics in a horizontal plane but little directivity in vertical planes. These antenna systems are coupled to individual input circuits of a lobe switch 29 having a common output circuit coupled to an input circuit of a receiver 3S. The modulation signal developed in the output circuit of the unit i2 is applied to a gain-control circuit of the receiver 30 for a purpose presently to be explained. The output circuit of the receiver 3c is coupled through an amplifier and phase inverter 3l to a pair of horizontal deflecting electrodes, designated as H, provided in a cathode-ray tube 32. The output circuit of the receiver E@ may also be coupled to a pair of earphones P if desired to provide audible indications that one or more reply Wave signals are being received. The responser 26 also includes a scanning-signal generator 33 having an output circuit coupled to a pair of vertical defiecting electrodes, designated as V, provided in the cathode-ray tube 32. A synchroniaing-signal crcuit of the generator 33 is coupled to an output circuit of the generator H to be synchronized in operation by the pulse signal generated by the latter. The responser 2e also includes a lobe-switch generator 3ft having an output circuit coupled to a control circuit of the lobe switch Z9 and to a control circuit of unit 3f.

Considering new the operation of the conin munication system just described, and referring to the curves of Fig. 2, the pulse generator it generates a signal of periodic-pulse wave form, as represented by curve A, and applies a pulse of this signal at time to to the paired-pulse g era-- tor ii. The detailed operation of the latter will be considered hereinafter, out it may be stated for purpo es of the present description that the generator I2 operates to generate a signal of pulse wave form having repeated pairs oi pulses, as represented by curve B, of which each pair 0i pulses a spacing c between the leading thereof varying with the control action provided by the altimeter i3. The latter for centen ience ce considered a barometer which provides a control action the magnitude oi uf'hich varies with the ambient barometrie pressure. Assuming that the interrogator ES and responoer Zii are carried by an aircraft, the control action provided by the altimeter it and the corresponding spacing a. between the paire-:l pulses of the signal generated by generator l2 varies with the altitude of the aircraft. This signal ot the generator i2 is applied to the modulation input circuit of the wave-signal generator i5 to modulate the wave signal generated by the latter. The modulated Wave signal is radiated by the antenna system i ii.

The transmitted wave signal ci the interroga tor l0 is received by the receiver ii of the distant transponder it! and the pulseminoduiation co2-nw ponente thereof are derived by the receiver applied with negative polarity, as represented by curve C, to the input circuit of the decoder The operation ci the decoder 2E will considered in detail hereinafter, but for purposes cf the present description it may be stated that it has an operation initiated in time relftionship with the occurrence at time t1 ci the lea ing edge of the irst pulse ci each derived pair or" pulses. The operation of the decoder E@ is so controiled by the altirneter 2i that the decoder generates and applies to a signal-translating s de inciuded in the decoder a signal ci pulse v Ecrin, as represented by curve D, having leading occurring an interval tz after the decoder 229 is initiated in operation. The interval te; with the magnitude ci' the control action provided by the altimeter 2i. Assuming that transp-onder i9 is carried by an aircrA t, the altimeter 2! may be simply a barometeso that the time interval t2 has a value varying with the t e assumed condiJ altitude. of .tl'ieaircraft The signalftranslating I`:stage .included 'in .the decoder is .arrangedto nslate to the .reply-signal generator the fisecond pulse of each pairderived in thecutput circuitof the receiver il, but biasedithat 'the stage translates suoli pulsesonly Y'when there 'is Vapplied thereto a pulse of the signal .by the. decoder Eil, such as the pulse represented bycurve D. The signabtranslating stage is thus conditioned to te asignal during thev time. interval t3 and, hence, conditioned to translate tothe generator 22 the second pulse of each pair ,derived in the output" c c 't of thereceiver il if corre proximately interval.' This would occur, under is, when. the altimeter of' the. interi' i s at the the altimetcr of the transponder le. Assuming that the secr l pulse of derived pair is translated b ne decoder il there is ap -lied to `the replj';-signal generator a signal. of pulse- Wave for as epresented by curro -l l1 pulse of this appli signal initiate-s the generae tion of a correspe fg pulse or ci pulses of a reply siffn le particular Wave forni ol? the reply s iai, for Vsample, the number, dura tions and spasings of pulses thereof,

adequately identifies the aircraft which carries the transponder l or the bearing ci suon airM This signal, represented by curve craft, or both. F, is applied through the amplifier 23 modulation input circuit of the were* erator .2t to modulate the wave signal latter. The modulated wave signal is radiated. by the antenna as a reply-modulated wave signal.. The ro' ly signal ci generator 22 is also applied to a g .ln-oontrol circuit oi the recel/er or" operation rreously connus, that the under control of tite sidered at this poi t. operation or the decoder altimeter 2 l, is such that the leading edge ci each. pulse of the signal generated thereby occurs with a time interval t2 or 2132" after the initiation of rZhis signal operati-cn or decoder at time t1.

the trave represente "cn-line curve or D. apparenu that scc id ci 'n p pulses ed l l .tput circuit ci" the rec.`

by the translating sta the not proper conditioned to t1 nsla-te during the occurrence 'No reply wave signal is consequently "lined by the transponder' Such a condition would for example, when the aircraft carrying the t1 ponder lil is an altitude either above or below that of the aircraft 4tvniclrcarries the ntcrrogator it.

The reply wave signal of the transpondor it is received by the antenna systems 2l' and 28 of the respcnser 2S and is applied to the lobe switch 29. The lobe-switch generator generates a signal which so `controls the lobe switch 23 that the antenna systems 2l' and 28 are alternately connected to the input circuit of the receiver 3). Since the antenna systems 2l and 28 are directive, the intensity of the Wave signal received by the antenna 2l is the same as that received by the antenna 23 only when the transponder i9 is carried by a craft which is positioned directly le Usl.

-enengy "of the ,interrogator transmissions.

antenna 16.. of the interrogator lll. While this .energy is likewise applied through the lobe switch 29 .to the receiverv Sli, there .isp-applied at thesame time vto again-control circuit of the receiver the modulationsignal developed in the output circuit of the ypaired-pulse generator l2 of the interro- Agator. Thismodulation signal so controls the gainv ofthe receiver4 Si? that the latter does not y translate the Waver-signal energy directly received by the antenna Isystems 2l and 23 from the in- -terrogator antennajl thus to yprevent possible paralysis A.of the receiver. byy the directlyv received The receiver 3G .derives the modulation components of the received Wave signal and applies .them

through the amplifier and phasey inverter 3l to the horizontalydeilecting electrodes H of the cathode-ray tubeZ. VrEhe signal generated by the lobe-'switch generatortll so controls the unit. Si that these Amodulation components are kapplied alternately to the tube 32 with direct phase and inverted phase in synchronism with the alternate connection of the antenna systems El and 28 to the receivery 36. Conseouently, one polarity of the modulation signal applied to tube 32 corresponds to reception by the antenna system 2l While the inverted phase of the applied modulation signalv corresponds to reception by the antenna system 28. By rotating the antenna sysm teins 2l and 28 until the modulation components of direct and inverted phases have equal amplin tudes, the azimuthk of the replying transponder may be' readily ascertained. There is also apu plied to the vertical deecting electrodes V of tube 32 a scanning potential of saw-tooth Wave formgenerated by the scanning-sienal generator 3,3 the operationkof which is synchronizedby the signal generated by the generator Il. The periodicity of this sfanning potential is thus the same as that of the signal of generator li, but the duration of the saw-tooth component durine each cyclethercof generally is much shorter than the period of the scanning potential and vhas a value dependent upon the desired maximum distance range of operation o? the communication s'stem. to the vertical defectine electrodes of tube 32 produces a vertical scanning movement of the electron beam of this tube while the modulation lcomponents applied to the horizontal deecting electrodes of the tube produce a horizontal deflection of the 4beam, thereby to provide the desired indication.V

An illustrative indication provided by the cath- The s"anning signal applind oderay tube 32 yis shown in Fig. 3 wherein it is.

assumed that the initiation of each vertical trace isat the bottom margin of the uorescent screen. A irst reply pulse vP .occurs a distance d from theA initiationrotthe vertical trace and is positioned asymmetrically With relation to the trace. The pulse P thus provides anindication that tlie vThe duration of pulse P may'provide an indication of fthe identityof the aircraft which carries the replying transpondor or, alternatively, may indicate that the yaircraft is ying in a direction lncludedrwithin a predetermined compass quadrant. There is also lshown in Fig. 3 a pair of reply pulses P' positioned a distance di from the initiation of the vertical trace and symmetrical with respect thereto. This indicates that a second rep-lying transponder is located a distance di in miles from the interrogator-responser and lies directly ahead of the antenna systems 21 and 2B, the grouping of a relatively long pulse fol-- lowcd by a relatively short pulse indicating aire craft identity or its direction of night.

In practice, it is preferable that the Wave sie'- nal generated by the generator I of the interrogator li) and received by the receiver il of the transponder IS have a frequency different from that generated by the generator 24 of the transponder and received by the receiver 39 of the responser 2li. The use of diferent transmission frequencies in this manner has the advantage that the receiver of the responser 2S dce-s not receive pulses of wave-signal energy transmitted by the interrogator il) and reected from such iixed or mobile objects as are capable of reflect ing Wave-signal energy. Such reflected pulses, if received, would be disturbing to the pilot of an aircraft even though they were readily distinguishable from the desired replies of distant transponders.

When the system of the present invention is used as an aircraft navigation and traffic-control system, it is preferable that the altimeter i3 of the interrogator i@ and the altimeter 2l of the transponder I9 be adjusted and sea-led at the factory to read absolute height above sea level at normal baremetric pressure. This ensures thatl all aircraft react identically and thus that aircraft flying at the saine height on a given day will be similarly altitude-coded regardless of curr rent barometric conditions.

While it is assumed in the foregoing description of the system operation that the interrogator-responser unit and the transponder unit were each carried by individual aircraft, it will be apparent that either the interrogator-responser unit or the transponder unit may be terrestrially located.. Where this is done, a fixed altitude difference is provided between the control of the altimeter and the unit controlled thereby, in a manner more fully explained hereinafter, by which the terrestrially located unit of the system may operate with an artificially injected altitude compensation which takes into account the difference in elevation between the terrestrial location at the point of installation and the air tramo acne used by aircraft with which it is desired to communicate.

It will also be apparent from the foregoing description of the invention that the pairedpulse generator i2 of the interregator i@ and the decoder 2li of the transponder I9 operate eiec tively as an altitude-controlled delay circuit responsive to a rst pulse applied thereto to generate or provide a second pulse delayed in point i ti ne with relation to the first pulse.

Of the signal generated by the generator l2 of the interrogator lil, as represented by curve B of Fig. 2, it will be apparent that the leading edges of the successive pulses of each pair thereof comprise at least two relatively variable waveform portions designating by their relative amount of variation, that is by their relative spacing, an individual one of a plurality of train-c zones available to mobile objects. Similarly, the signal derived in the output circuit oi the receiver il', and represented by curve C of 2. has a waveform corresponding to that of the signal last mentioned and consequently also includes two relatively variable Wave-forni portions, namely, the leading edges of successive pulses of each pair, which vary with those of the modulation signal generated by unit l2 and consequently establish by their relative amount of variation a particular one traiic zones.

The circuit arrangement of the paired-pulse generator l2 of the interrogator l0 may be as shown in Fig. 4 wherein elements correspondn ing to similar elements or" Fig. l are identified by similar reference numerals. The paired-pulse generator broadly comprises a univibrator 36, a

I. pulse former Si, a polarity inverter 3S, and a combining amplifier 35. The univibrator is seinetimes referred to in the art as a one-shot multivibrator, that is a relaxation oscillator normally having a stable operating condition but which may have an unstable operating condition initiated by the application of a control pulse and terminated by the discharge of a condenser in a time-constant network in the oscillator circuit. The polarity inverter SS has an input circuit coupled to an output circuit of the pulse generator Il and has an output circuit coupled to an input circuit or' the amplifier rihe univibrator 36 includes a triode vacuum tube 3s have ing input electrodes coupled to the output cir cuit or' the pulse generator il and having output electrodes coupled to the input electrodes oi' a second triode vacuum tube fill. Connected be tween the control electrode and cathode of the vacuum tube tl is an adjustable resistor il having a movable contact d2 which is pivoted at d3 and is mechanically connected to the altimeter 53, shown as an aneroid barometer, controlled` adjustment thereby. The cathode circuits of the vacuum tubes 39 and lill include a common cath 0de resistor 46. The output electrodes of the vacuum tube llt are coupled to the input electrodes of the vacuum tube through a small condenser 35. A switch it is provided, for a purpose presently to be explained, selectively to connect the control electrode of tube Sii to the cathode of the latter eitherl through the resister 4| or through a manually adjustable resistor al. A manually adjustable control it is provided to rotate the resistive element fil relative to its Contact d2 for a purpose presently to be described.

The pulse former 37 includes a vacuum tube I 5B having input electrodes coupled through a condenser 5I and through the condenser la to the output electrodes oi" the vacuum tube or" the univibrator 3E. rIhe control electrode of vacuum tube 59 is so biased through a resistor 49 from a source of negative potential, indicated as 0, as to render the tube 5U normally nonconductive. The anode of the tube 5b is coupled through a primary Winding 52 of a transformer 53 to a source of potential, indicated as +B. The pulse former 3l includes a second vacuum tube 54 having output electrodes coupled in par allel with the output electrodes of tube li@ and having a control electrode biased from a negative potential source, indicated as 01, through a resistor 55 and a secondary winding 56 of the transformer 53. A conventional delay line lli is coupled across the resistor 55. The pulse former 3l' includes a third vacuum tube 53 having a control electrode biased from a negative potential source, indicated as -Cs, through tertiary Winding 5d of the transformer The output load impedance of the vacuum tube of the plurality ofl il comprises a cathode resistor The `latter is coupled to an input circuit ofthe amplier 55. lConsidering novv the operation oi the pairedpulse generator just described, and referring to the curves or" Fig. 5, one pulse of the signalfct pulse waveform generated by the pulse generator il is represented by curve Cf. The pulses ofv this signal are applied to the pairedepulse generator with negative polarity as-indicated. T- is signal is applied to the polarity invertert which inverts the polarity or the pulses and applies a signal havinnv positive polarity pulses to an input circuit'oi the amplifier The signal of the generator i l is also applied to the input electrodes of the vacuum tube of the univibrator The control electrode or this tube normally has little or no bias with respect to its'associated cathode so that the tube is normally conductive. The space current oi the tube news through its catho e resistor it to'developthereacross -a Ypotential oi sufficient magnitude to maintain the vacuum tube normally biased to anode-current cutoi'.

When a negative pulse of the signal-of generator il is applied to the input electrodes of vacuum tube 3S, it reduces thefanode current-of this tube thereby applying from the output circuit of tubeil to the control electrode of vacuum tube lil a potential -suciently lpositive to initiate anode-current iiow in the Ylatter tube. This in turn applies a negative potentialtnrough the condenser l5 to the control electrode for vacuum tube 39 further toreduce'the anode'current of the latter. This action is cumulative and the vacuum tube til is rapidly driven to anodecurrent cutoff While the vacuum tube kit quickly reaches a condition of full conductivity. rllhe voltage-applied to thecontrolelectrcde of the vacuum tube te begins to go 'more positive as the condenser discharges through a circuit which includes the resistor il andthe anodecathode space of tubetl. A value of biasis eventually reached after a vcertaintirne interval which permits anode-current'cw to'be'initiated'in tube 39. When this occurs, a `negative potential is l applied from the output circuit of tube 3Q to the control electrode of the tube iid, thereby to "de crease the anodecurrent of the-latter. This in turn applies a positive potential from the out put circuit of tube lill to the ccntrol'electrode'of tube 39 further increasing the anodecurrent ci the latter. 'ihisaction is-again cumulative until vacuum tube is rapidlydriven to anode-current cutoff and vacuum tube tiquickly reaches a state ci fullvconductivity.

The anode potential of the tube-4G during the cycle of operation described is represented-inliig. 5 by curve H. It will be apparent that this potential variation is of pulse Wave form having a pulse duration dependent upon the rapidity with which the condenser e5 discharges through the resistor ill. This pulse interval is therefore adjustable by adjustment of the `value 'of 'the resistor lll. The latter, however, has a lvalue "of resistance controlled by the -altirneter vl-t and thus a value ci resistance rvaryingr with the ambient barometric pressure. From 'thisit 'will be apparent that the durationci each pulse-cf the potential developed in the output'circuit of the vacuum tube all hasa duration 'controlled by the altimeter I3.

This potential is applied through the condenser 5l to the input electrodes of the vacuum rtube di). The condenser 5l and the grid resistor Il@ of the tube 56 differentiate the .applied potential pulse to develop a potential iof `pulse :Wave -.r"o'rm, represented by curve il, having aine'gative polarity pulse vfollowed by a positive polarity Vpulse correspending respectively to the leading'and lagging edges of the appliedpotential pulse.V The vacuum tube Ell is normally biased to anode-current cutoil from the source of potential C Vso that the negative polarity pulse hasrno erle-ct "on the operation of this tube. The positive polarity pulse, however, initiates `'anode-ourrent new vin the tube 5t and this current in flowing through the primary Winding 52 of the transformer. e induces a voltage inthe 'secondary uf ling thereof. This induced lvoltage is applied u di positive polarity to the control electrodeof the vacuum tube 5ft to initiate -ahod'emcurrent ov through the latter. The control electrode'otube et becomes 'sufficiently `positive thaty it draws cui rent to developacrcss the resistor d5 va negative impulse which is applied to the input terminals of the delay line 5?. The vacuum tube ed 'eea-sesto pass current at-the end of the positive pot" pulse applied to its control electrode, 'l transformer 53 has `sufoient inductanoefthat the anode current ci tube '35i-i cannot rise to its maximum value for a short time interval. At about the end of this interval, the impulse which earlier was applied to the delay line'il' yhas traveled to the end of the line andris reiected Without reversal ci polarity to the input terminals thereof Where it is applied to the control electrode of vacuum tube 5d to bias this tube to anode-curA rent cutoff. The interval during which 'anodecurrent flows in the vacuum tube '5ft thus determined by the interval required `for an impulse to travel down the delay line 5'! `and to return by reflection to its input terminals. Aidesired value of time delay canbe provided by a suitable choice vof the values of the circuit components of the delay line di in well-known manner. The operation described requires that the reflected impulse have the saine polarity as the impulse applied to the input terminals or delay line 5l. For this purpose, the remote end of the delay line is open-circuited.

The anode'currentsof the tubes Ell and lili in owing throughthe primary Winding 52 or" the transformer '53 induces in the tertiary winding E9 thereof a potential of pulse Wave form, represented by curve K of rig. 5. it will be ap parent that the duration of each pulse of this potential has a value determined vby 'the value of delay provided by the delay line til. This potential is applied With positive polarity to the input electrodes of the vacuum tube 58, which is normally biased to'anodeurrent cutoff from the source of bias potential 03, and each pulse thereof initiates anode-current now in this tube. There is thusy developed across the cathode resistor 6e of this tube a potential of pulse Wave form, represented by curve L, and this potential is applied to an input circuit of the amplifier 35. Y Consequently, there is 'applied to the amplier 35 Vat Atime-t0, Fig. v5, Lirst pulse of positive polarity developed in the output circuit of the polarity inverter Vat time ta a second pulse ci positive polarity developed across the cathode resistor 5a. The composite signal applied to the amplifier thus has thewave form represented by curve B of-Fig. 2.

It will be apparent from what has been said that'the time interval trl-ta, and the resultant spacing a between the pulses is established by the duration of the potential pulse developed in theout'put circuit of the Vacuum tube 40 Vof the vunivibrator 36, this potential pulse being repthe amplifier t a signal of paired-pulse Wave form of which each pair of pulses thereof has a variable spacing controlled by the altimeter i3.

It was previously mentioned that Where the interrogator-responser units of the communication system are carried by an aircraft, occasion may arise Where the pilot thereof desires to explore the adjacent altitude zone above or below the one in which he is then flying in order to ascertain Whether, for example, it may be safe for him to increase or decrease his altitude to a position in the other zone. For this purpose, the Fig. e paired-pulse generator arrangement includes a manually operable switch d6 by which the control electrode of vacuum tube 39 may be connected to its associated cathode through the manually adjustable resistor dl. This permits the pilot manually to adjust the resistor 4'! to a value such that the pulse duration of the potential developed in the output circuit of the vacuum tube i9 corresponds to that representative of the altitude zone he wishes to explore. The paired pulses generated by the paired-pulse generator then have a spacing characteristic of the new altitude zone and are thus eiective to interrogato transponzlors carried by aircraft in that Zone.

Where the interrogator-responser units of the communication system are terrestrially located, it will be apparent that the barometric pressure at the altimeter thereof does not correspond to that for an aircraft iiying at some altitude zone, for example, five thousand feet. To permit interrogation of such aircraft by the terrestrially Ilocated interrogator-responser units, the pairedpulse generator of the interrogator thereof includes. as shown in Fig. 4, a manual adjustment 43 by which the resistor 3| may be rotated relative to the contact d2 associated therewith thus to insert an artificial altitude compensation. The compensation, once inserted at a given installation, remains thereafter fixed during the life of the equipment and consistent communication is maintained with aircraft at the selected altitude zone without regard to varying barometric pressures in that locality.

The arrangement of the decoder 29 utilized in the transponder i9 of the Fig. l arrangement is shown schematically in Fig. 6. This decoder utilizer a univibrator 36 and pulse former 3l as in the Fig. 4 arrangement and includes an ampliier 2 having an input circuit coupled t-o the output circuit of the receiver il and having an output circuit coupled to an input circuit of the reply-signal generator 22. The amplier includes a gain-control circuit coupled to an output circuit of the pulse former 31. The univibrator 35 is controlled by the altimeter 2l in a manner similar to that shown in Fig. 4. In the decoder, the potential of positive polarity and of pulse wave form developed in the output circuit oi the pulse former 3l preferably has a pulse duration a little longer than that of the second pulse of each pair developed in the output circuit of the receiver il' and applied to the input circuit o the amplifier E2. The desired pulse duration is established, as explained, by selection of the values of the circuit components of the delay line included in the pulse former 3l, The ampliier 62 is so biased and operated that it translates to its output circuit the signal applied to its input circuit only during each pulse of the potential applied to the amplier gain-control circuit from the pulse former 3l. Consequently, as explained in connection with Fig. 2, if each pulse of the potential applied to the gain-control circuit of the amplifier 62 from the pulse former 3l occurs with the proper time interval t2 following the leading edge of the irst pulse of each pair developed in the output circuit of the receiver Il, and represented by curve C of Fig. 2, the second pulse of each such pair is translated by the amplier 62 to develop in the output circuit of the latter a potential of pulse wave form as represented by curve E of Fig. 2. The univibrator 36 of the decoder may have the manual adjustments, described in connection with Fig. 4, by which the transponder may be enabled to respond to manually selected altitude zones, oi' by which the transponder when terrestrially located may be provided with an artiiicial altitude compensation.

Fig. 7 represents schematically a portion of an interroga-tor embodying a modified form of the invention which is essentially similar to that of Fig. l, similar elements being designated by similar reference numerals. The paired-pulse generator i2 of the Fig. l arrangement is replaced in the present interrogator by a univibrator 36, similar to the univibrator 35 of the Fig. 4 arrangement, and a polarity inverter 63 which are coupled in tandem between the pulse generator l i and the amplifier 3%. As explained in connection With Fig. 4, the univibrator 35 develops a potential of pulse Wave form the duration of each pulse of which is controlled by the altimeter i3. Consequently, this potential may have its polarity inverted by the unit 63 for application as a modulation signal to the amplifier 35. In this modification, the carrier wave transmitted by the interrogator is thus modulated by a signal of pulse wave form, each pulse of which has two relatively variable wave-form portions designating by their relative amount of variation under control of the ailtimeter it an individual one of a plurality of trallc zones available to mobile objects. 'I' he two relatively variable Waveform por tions of this modulation signal are, of course, the leading and lagging edges of each pulse thereof, the spacing between the leading and lagging edges of each pulse being controlled by the altimeter i3 thus to designate an individual altitude traflic zone.

Fig. 8 represents a portion of a transponder suitable for use with an interrogator embodying the Fig. 7 modified form of the invention. Elements in Fig. 8 corresponding to similar elements of Fig. 1 are designated by similar reference numerals. In the present arrangement, there is derived in the output circuit of the receiver i 'i from a received modulated'wave signal a signal oi' pulse Wave form, each pulse or which has a variable pulse duration as established by the altitude control or the interrogator which transmitted the Wave signal. This derived signal is applied to a diierentiating network 613 which derives for each such pulse a negative and a positive polarity pulse corresponding respectively to the leading and lagging edges of each applied pulse, the operation in this regard being similar to that described above in connection with curves H and I of Fig. 5. This pulse signal derived by diierentiation is applied to the decoder 2t which translates the second positive polarity pulses thereof i'ffeach pair of suchpulseshasla'spacing corresponding to that established bythe altimeter 2l, thecperation in this respectlbe'ing essentially siinil .i to that described inrconneetionavithie. 6.

Fig. 9 represents schematically alcomplete in terrogator lll embodying an additionallyrnodi i'ied forin or the invention essentiall-ysirnilar `to the l arrangement, 'elernentsa of Fig. i 9 Ycorrespending to si'nilar elements of Fig. l being-1 designated by similar reference numerals. vln the present arrangement, thepaired-pulse generator and altiineter 'ci' the `interrogator yare 'comprised by'a transmitter Stand a receiveri'l. .The 'signal generated by the'generator VVil .isapplied to .the amp lier also to a. modulation lcircuit io the transos lto"modulate the rWave vsignal generated by the latter. This iruzclulatedrvvave signal is radiated by an antenna 5E provided for the transmitter i t5, vthe antenna 'e8 v`having a .directive characteristic ldirected rdowmvardly Kto- Ward the ground. The units 5E andzlil' essentially comprise a radio altimeter and Wave-signalpulses rellectesl ground are received by a receiving antenna provided for the `eceiver el. The inodulationvccmponcnts ofthe received wave signal are derived and applied to the amplierf. rlhere is consequently applied to .the latterfrom the pulse generator l l andthe receiver 6l acornposite signal of pulse fwave form having paired pulses. Each pair 01' Apulses of this Vcomposite signalhas variablespacing dependent upon the time interval required for a pulse of Wave-signal energy to travel from the antenna dil `to the ground. and to return to the antenna, andfthus a spacing varyinf,lr with the altitude-0f theunits 53 and t? with relation to the ground.

Fig. l0 represents a portion of atranspondor suitable for use Ywith the interrogator oFig. ,9 and utilizing a similar radio .altimeter 'The modulation signal developedin the output circuit ci the receiver il applied toa modulation circuit of a transmitter il which transmits a pulseniodulated 'Wave signal vdirected toward "the ground. Paises oi' Waveesignal energy reflected from the ground are received by a receiver 'l2 .and the modulation components thereof are .applied as control potential pulses to the control circuit of the 'er rlhese control vpotential pulses con .ition the amplifier to'translateduring eachpulse thereof the signal 'appliedto'theamplier from the recei il. Theoperati'on of the ampli-ner or in this regard is essentiailysiinilar to that ci the amplifier o1" 'theFi-g. 6 arrangement. The use or a radio altinre'ter in an interrogator, as in S, and in a transponder, as in Fig. lo, is prei'erable only under 'conditions Where the ground suriace no severe undulations, as over flat country or over the surface of the ocean. The reasons for this are apparent when it is conthat the altitude control of rvthe interrogator and responser should beuniform Withlal'titude and without regard to the surface fof the terrain over which two'widely spaced intercom- 'znunicating aircraft may be flying at lafgiven nrw rnent.

l" 11 represents schematically a 'portion offa transponder suitable for use ina-n. aircraft navi gation system embody-loathe present invention to provide indications or Ythe presence o an ob stacle with which aircraft in ight may collide. Such obstacle indications may, for examplebe furnished for tall buildings in a city, for broadcast antenna tov/"ers, or Vfor mountain peaks. This arrangement is essentially'similartotl' t of Fig. 6, similar elements being designated oy similar reference numerals, exceptthat the pulse ormer used in Fig. 6 is replaced in the instant arrangement by apolarity inverter l Whichfhas an input circuit coupled to thevoutput'circuit-oi the univibrator 36 andinas 'an output-circuitcoupliedto a'contro'l circuit of the amplier 52. Additionally, in thepresent arrangement the univibratorvilt vhas a pulse duration corresponding to the altitude of the'highest point ofthe obstacle. Thus, when the potential of pulse'vlave orrngenerated bythe univibra-tor .3.5 is l'reversed in :po larity by the Sunit l 'and lapplied .to 'the control circuit .of the aniplier @2, :the 'amplifier is eriabled 'to translate all signals applied thereto from the r, ceiver il'whichihavefaivalue ci separation between the 'pulses ci "each Y.pair 'thereof .correg spending to any altitude less vthan that represented by the duration'ofieaolrpulse of'the uni'- vibrator 3S. A signal'having a pairedepulse'spacing greater thanthis value, and'correspondingito a higher altitude, does not fall'within the :range of altitude values established bythe pulsesgenerated :by the univibrator `3ft and, .consequently, is not translated by the 'amplifier t2. This only means; of: course, that an aircraft transmitting an interro-ating signal With this wider paired-pulse spacing-isat ahigher altitude than the obstacle and-.may'passsafely over it so that no 'obstacle indication should be furnished 'to such aircraft.

1t will be apparent from the vforegoing description of the invention that a wave-signal communication system embodying the invention has a wide range or' application in traffic-control systems and particularly inthe navigation and traffic control of aircraft. `When used with air craft, it materially aids in the'navigation of'aircraft along'a prescribed course While providing positive indications of all `suchhazardous condi tions as may `resultrin thecollision of two aircraft in flight .or the collision oi an aircraft with an obstacle. lThe Vsystem of .the invention permits the progress of an aircraft along aprescribetl course to be-readily-ollowed by atraiiiccontrol center and enables positive yidentification 'at-the centerof suchva-ircraft. .Additionall the communication system of Ythe invention Vhasthe advantage .that it permits positive and safe altitude separation of aircraft so that prescribed coursesrnay be safelyiollotved by'largenurnbers of aircraft even n,though a 4plurality ,ot such courses cross one anothcror converge into one or more altitudezones While there-have been described 4what-area?l present considered to -betlie preferred embodiinentsofthis invention, it willbe obvious to those skilled .in the art that yvarious changes Yand inorliications may vlee made therein Without-departing from the invention,.and it is, therefore, aimed tocover all such changes and modificationsas iallwithin the truefspirit and-scope .of the. invention.

What is claimed is:

l. A system tor communication -between two spaced Wave-,signal stationsat least oneof which is .carried by amob-ile object comprising: a first altitudenre ponsiverdevice at one or said stations for developing .a irst control .effect .representative v'of altitude; means at .said one station responsive to said control effect for transmittingr Wave-signal energymodul'ated bysignal energy otpulse Wave orrn havingat'leasttwo relatively Variable edgeportions designating bytheirrelative amount ofvariation an individualone of a plurality of vertically positioned trahie zones available to Vmobile objects; ymeans at the vother of said stations for receiving said modulated Wave-signal energy and for deriving in response thereto signal energy having two relatively variable edge portions related to those of said modulation signal energy; a second device at said other station for developing a second control ei"- fect representative of its altitude; and control means at said other station responsive to said second -control eiiect for utilizing said derived signal energy only when said edge portions thereof have 'a relative amount of variation lying Within a selectable range of values automatically established by said control means under control of said second altitude-representative device as designating a selectable one of said plurality of traiic zones.

2. A system for communication between two spaced Wave-signal stations at least one of which is carried by a mobile object comprising: a iirst barometer for providing a control action varying with barometric pressure, means at one of said stations and including said rst barometer for transmitting wave-signal energy modulated by signal energy of pulse wave form having under control of said barometer at least two relatively .y

variable edge portions automatically designating by their relative amount of variation an individual one of a plurality of vertically positioned traffic zones available to mobile objects; means at the other of said stations for receiving said modulated wave-signal energy and for deriving in response thereto signal energy having two relatively variable edge portions varying with those of said modulation-signal energy; a second barometer for providing a control action varying with barometric pressure; and means including said second barometer and a control means at said other station having a cycle of operation initiated in a predetermined time relationship with the occurrence of one of said edge portions of said derived signal energy for utilizing said derived signal energy only when the other of said edge portions thereof has a relative amount of variation lying within a selectable range of values automatically established by said control means under control of said second barometer as designating a selectable one of said plurality of vertically positioned tramo zones.

3. A system for communication between two spaced wave-signal stations at least one of which 'Y is carried by a mobile object comprising: means including an altitude-responsive device at one of said stations for transmitting wave-signal energy modulated by signal energy of pulse wave form having at least two relatively variable edge portions designating by their relative amount of variation an individual one of a plurality of altitude trafc zones available to mobile objects; means at the other of said stations for receiving said modulated wave-signal energy and for deriving in response thereto signal energy having two relatively variable edge portions related to those of said modulation-signal energy; means including a control means at said other station responsive to one of said edge portions of said derived signal energy for transmitting a replymodulated wave signal in response to said derived signal energy only when the other of said edge portions thereof has a relative amount of vari-ation lying within a selectable range of values established by said control means as designating a selectable one of said plurality of traiic zones; means at said one station for receiving said transmitted reply wave signal and for deriving the modulation components thereof; and means for utilizing said derived modulation components to indicate the distance between said stations.

4. A system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object comprising: means including an altitude-responsive device at one of said stations for transmitting wave-signal energy modulated by signal energy of pulse wave form having at least two relatively variable edge portions designating by their relative amount of variation an individual one of a plurality of altitude trailic zones available to mobile objects; means at the other of said stations for receiving said modulated wave-signal energy and for deriving in response thereto signal energy having two relatively variable edge portions related to those of said modulation-signal energy; means including a control means at said other station responsive to one of said edge portions of said derived signal energy for transmitting a replymodulated wave signal in response to said derived signal energy only when the other of said edge portions thereof has a relative amount of variation lying within a selectable range of values established by said control means as designating a selectable one of said plurality of tranic zones; means at said one station for receiving said transmitted reply wave signal and for deriving the modulation components thereof; and means for utilizing said derived modulation components to indicate the distance between said stations and the direction of said one station from said other station.

5. A system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object comprising: means including an altitude-responsive device at one of said stations for transmitting wave-signal energy modulated by signal energy of pulse wave form having the leading edges of two successive pulses thereof variably spaced to designate by their relative amount of spacing an individual one of a plurality of altitude. traiilc Vzones available to mobile objects; means at the other of said stations for receiving said modulated wave-signal energy and for deriving in response thereto signal energy having a pulse wave form corresponding to that of said modulation-signal energy and including two variably .spaced successive pulses; means including a control means at said other station responsive to the leading edge of one of said pulses of said derived signal energy for transmitting areply-modulated wave signal in -respense to said derived signal energy only when the leading edge of the other of said pulses thereof has a relative amount of spacing lying within a selectable range of values established by said control means as desigating a selectable .one of said plurality of traffic zones and means at said one station for receiving said transmitted` reply wave signal and for utilizing the modulation components thereof to provide at least an indication that a reply wave signal has been received.

6. A system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object comprising: means at one of said stations, including a device for developing an eiTcct representative or" altitude, responsive to said eiect for transmitting wave-signal energy modulated by signal energy of pulse wave form having at least two relatively variable edge portions designating by their relative amount of variation an individual one of a plurality of altitude traffic zones available lto mobile objects; means at the other of said stations for receiving said modulated Wave-signal energy. and for; de.- rivingY in response thereto vsignal. energy.l having two relativelyA variable edge portions related' to those of said modulationfsignal energy; means including'v a controlmeans .atl said otherfstation for transmitting a reply-modulated.wavesignal in response to said derived signal energy. only Whensaid edge portionsthereof' have a` relative amount of variation lying Within. a selectable range of values. established by said. control means asl designating a selectable oneof saidplurality of altitude trafc zones; and means at said. one station forl receiving` said.transmitted.jreplyy Wave signaland for utilizing themodulationcomponents thereof to provide at least lan indication. that a reply .wave signal has. been received,

'7, A system for communication between.. two spaced wave-signal.y stations at least one. of .which iscarried by a mobileobject.; comprising: a. vfirst device at 1 onefof said'stationsffor developing. a

first control' effect representativeA of altitude; means at saidone of said.stationsresponsiveto said control effect for transmitting. wave-.signal energy..rmiolulated` by signal energy of pulsewave formhaving atleast tworelativelyvariable edge portions.. designating by. their relative amount. of variation ansindividual one of a, pluralityof alf titude traffic zones. available tomobile objects; means at theA other of said. stations for. receiving said modulatedwave-signal energy andfor-def riving` in response thereto signal energy.. having two relatively` variable edge. portions. related.. to those of said modulation-signal energy; a second device at said other stationfor. developing a second. control effect rtl?srefserltative.of its alti.

tude; control means .at saidvother stationresponsive to said second control effect .for utilizing said derivedE signal energy. only 'vs/l'lenA the. said. edge portions thereof have a relativev .amount ofvaria.- tion lying within a selectable g rangelv of v alues automatically established byl said. controlineans.

under control of said second.altituderepresentative device as automatically..designating a.select.- able .one of said plurality of altitude traiiic zones; andymeansat saidonestation for receiving said transmitted replywavesignal andfor. utilizing the `modulation components thereof to Yprovide at least an. indication that a.. reply waversignalhas been received. A

8.I A systemior communication. betweentwo spaced wave-signalstationsat leastoneof .Which isl carried. by a mobile object4 comprising: afirst device. at one of said stations. for. developingI a first control effect. representativeof altitude; means at saidonestation responsive .to ',saidcone trol eiect. for transmitting waversignalenergy modulated bysignal. energy. of; pulsewave .form having the leading. edges.ofjtworsuccessivei pulses thereof variably spacedtodesignate byy their. relativeA amount of spacing. an individual. one. of; a`plurality ofv vertically.'y positioned. altitude traffic 'zonesV availablew toA mobile objects; means. atA thegother of saidl stations. forreceiving said modulated. Wave-.signal energy. and. for deriving in response theretosignal. energy. havinga pulse wave form corresponding to that of: saidmodula tion-'signal energy. and.'including. two variablyA spaced successivepulses; aseconddevice at said othery station for developing a secondcontrol effect representative of its. altitude control means at.y said. other. station responsivey to.said.second control, eifect and. .to the. .leadingfedgefof vthe-first offsai'dpulses-of saidderivech signal energyf.for;V I

utilizingV the. second oisaid-` twdisuccessive. pulses of said '.derived.. signal energy. for initiatingV` the.

2i) transmissionof a.. replymodulatem waversignal only when the.. leading. edge .of .thesecondlofg Said pulses ofsaidderived signalenergy hasfa relative amount of. spacinglyingwithin a selectablelralige ofvalues establishedby said controlmeans .under control of said second. altitude-representative device. as. automatically.- designatingv a selectable one of said plurality; of traffic zones; andlmearis at said onestation for .receiving said .transmitted reply .wave signal and for utilizing. themodulaf tion-f; componentsv thereof to. provide. ati least. an indication. that a. reply wave,signa1. has..been received.

9.. As system forf communication betweenf two spacedwave-signal-.stationsat. least onev offwhich is carried by a mobile objectcomprising: means including an altitudefresponsive.device.at one; of said-stations for transmitting wave-signal :ener-gy modulated. by; signalf` venergy of.r pulsefwave. form havingY theY leading edges of tW-o successive pulses .thereof variably spaced; under .,-controlsof said device automatically; toi. designate by their r-rela..- tive-:amountof-..- spacing Y an individualronersof, a plurality-of.l altitude trai-.e zones availablevto mobile. objects.; means -at`theother of; saidf'staf tions. for; receiving.- said modulated wave-signal energy; and for derivingy in response. theretosignal energy -havingiafpulsewave Aform corresponding to ...that of f said', modulations-signal f energy .includ-,.- ing-i said two variably spacedfsuccessivezpulses; means. including a second. `alt itu(ie-responsive device.. and a.; control means at.V said; other'- station havinga cycle. of operation initiatedeinga prede.,-Y termined time relationship.withthegleadingdge of.. .the rst of; said4 pulses of. said; derived:` signal energy. for utilizing .the lseccncliof 'saidtwodcrived successive pulsesvfor initiating:-ther transmission ofafreply-modulated wavesignalif only;v whensthe leading edge of thersecondfof-said derivedpulses has: a relativeamount-fof spacing.y lying Withinia:

selectable range.. ofj valuesrestablished. byf said control meansunder control of; said.. second altitudeeresponsivef device. as automatically desigs nating a selectable onegof-.said plurality ofialtitucletrafiic. Zones and meansA at .said one station for: receiving said transmitted reply .wave signal and for utilizing the modulation. components.. thereof to provide atleast an indicationthatra reply; wave; signaly has been received.

I0.y In afsystern.` fory communication between twoaspaced wave-signalstations at least onej-of which is `carried-bye, mobile object,` a-wave-sig'nal receiver... comprising: means for receiving vwavesignal energy modulatedrby signal-energy of pulse wave form having at least two relativelyvariabley edge' portions designating by their relative amountoff'variationan individual "one of a `pluralityoi" traffic zonesjavailableto-fmobile objects;l means responsive to said received-wavesign'al energy for: deriving signal.- energyI4 of pulse "Wave form corfespendingA tosaid modulation-signal energy,`

and including'v said: two variable edge portions,"

and vmeans including aunivibratorhavingfacycle` of'operation initiated in av predetermined time" re1at1onship with the occurrence ofj one ofv saidedgefportionsof said derived signaljenergy,A for utilizing saidderived signal energy only when the other of said edge portions thereof'hasarelative amount ofA variation. lying within` a selectable. f rangeofvalues established by said-univibratorfas selectively.v designating at least onel ofsaid-"rilli-A ralityof-trafiic zones. 11e-.Infl a; system for communication between two spaced; wave-signal stations:at least one Vv"ofv Whhlarried by1a mobileobject: azwave-signalreceiver comprising: means for receiving wavesignal energy modulated by signal energy of pulse wave form having at least two relatively variable edge portions designating by their relative amount of variati-en an individual one of a plurality of altitude traine zones available to mobile objects; means responsive to said received wave-signal energy for deriving signal energy having two relatively ve ole edge portions related to those of said modulation-signal energy; and means including an altitude-responsive device and including a univibratcr having a cycle of operation initiated in a predetermined time relationship with the occurrence of one of said edge portions of said derived signal energy for utilizing said derived signal energy only when the other of said edge portions thereof has a relative amount of variation lying within a selectable range of values established by said univibrator under control of said altitude-responsive device as selectively and automatically designating at least one of said piurality of altitude trafic zones.

12. In a system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object, a wave-signal receiver comprising: means for receiving wavesignal energy modulated by signal energy of pulse wave form having the leading edges of two successive pulses thereof variably spaced to designate by their relative spacing an individual one of a plurality of altitude trafiic zones available to mobile objects; means responsive to said received wave-signal energy for deriving signal energy having a pulse wave form corresponding to that of said modulation-signal energy and including said variable spacing between the leading edges of two successive pulses thereof; and means including an altitude-responsive device and a univibrator having a cycle of operation initiated in a predetermined time relationship with the leading edge of on-e pulse of said derived signal energy for utilizing said derived signal energy only when the leading edge of the other pulse thereof has a relative amount of spacing lying within a selectable range of valu-es established by said univibrator under control of said altitude-responsive device as selectively and automatically designating at least one of said plurality of tramo zones.

KNOX MCILWAIN.

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